The present invention relates to catalysts for selective reduction of nitrous oxides with ammonia and methods of use thereof.
Nitrous oxides, which originate during combustion processes, are among the main causes of acid rain or photo smog and the attendant environmental damage. In particular, nitrous oxides, along with fluorochlorohydrocarbons, are suspected of being responsible for the observed shrinkage of the ozone layer above the polar regions.
Sources of nitrous oxide emission are motor vehicle traffic, stationary combustion motors, power plants, thermal power stations, steam generators for industrial purposes, and industrial production facilities.
In power plants with boiler furnaces, of course, one can achieve a reduction in the nitrous oxide concentration in the waste gas by using very pure fuels or by means of optimization of the combustion systems. These are referred to as primary measures. However, these furnace-engineering measures encounter both technical and economic limitations. This is why additional secondary measures must be taken to comply with the legally prescribed emission limitation values. Such secondary measures to reduce nitrous oxides usually involve catalytic reduction methods, in which connection one uses mostly ammonia as the selectively acting reduction agent.
Many catalysts are already known for reductive catalytic lowering of nitrous oxide emissions. For example, German Patent Applications 12 59 298, 12 53 685, and 11 15 230 describe oxidic catalysts without noble metal, while German OLS 22 14 604 describes oxidic catalysts that contain noble metals.
German Patent 24 58 888 describes another catalyst which consists of an "intimate mixture" of the following components:
(A) titanium in the form of oxides; PA0 (B) at least one metal from the following group: B-1 iron and vanadium in the form of oxides and/or the group B-2 molybdenum, tungsten, nickel, cobalt, copper, chrome and uranium in the form of oxides; PA0 (C) tin in the form of oxides; PA0 (D) metals from the group of beryllium, magnesium, zinc, boron, aluminum, yttrium, rare earth elements, silicon, niobium, antimony, bismuth, and manganese in the form of oxides. PA0 1a: homogeneous solution method; PA0 1b: coprecipitation method; PA0 2: simultaneous use of solution and precipitation methods; PA0 3: precipitate-mixing method. PA0 (A) titanium oxide; PA0 (B.sub.1) at least one oxide of tungsten, silicon, boron, aluminum, phosphorus, zirconium, barium, yttrium, lanthanum, cerium and PA0 (B.sub.2) at least one oxide of vanadium, niobium, molybdenum, iron, copper,
The components are present in the following atomic ratios:
______________________________________ A to B to C to D = 1 to 0.01 through 10 to 0 through 0.2 to 0 through 0.15. ______________________________________
A catalyst with this composition is used for the reduction of oxygen-containing and ammonia-containing gas mixtures in the temperature range from 150.degree. to 550.degree. C. and at space velocity of 300-100,000 h.sup.-1.
These catalysts can be made by processes that are known in the art. However, these processes must always assure that the components (A) and (B) and optionally also (C) are obtained as an intimate mixture in the form of their respective oxides. The following are mentioned as typical examples of such production methods:
As preliminary stages or precursors of the components (A), (B), and (C), one uses solutions and/or precipitates, such as, for example, hydroxides or water-containing gels which are mixed to form an intimate mixture and which are then subjected to calcination. In the process, the precursors are pyrolized and one gets the desired intimate mixture of the oxides of the components that are critical for catalysis. The calcination temperature should be between 300.degree. and 800.degree. C. Below 300.degree. C. one cannot obtain an intimate mixture of the oxides and thus an active catalyst; above 800.degree. C., sintering takes place and that leads to the loss of the effective catalyst surface.
By way of an initial starting material for titanium as component (A), there can be employed, for example, various titanic acids, titanium hydroxide, and various titanium salts, such as halogenides, titanium sulfate, titanyl sulfate, and the like. Organic components of titanium, for example, titanalkoxides can also be used as initial material for titanium. Titanium oxide cannot be used in the calcined rutile or anatase form.
A further improvement of these catalyst production methods, described in German Patent Application 24 58 888, constitutes the basis of the catalyst material described in German Patent 35 31 809. Titanium dioxide is used as initial material for the preparation of this catalyst mass and this titanium dioxide is ground together with vanadium oxide and one or more oxides of the elements tungsten, molybdenum, phosphorus, chrome, copper, iron, and uranium. Afterward it is subjected to at least one thermal treatment step. Here, tungsten and molybdenum are completely or partly replaced by phosphorus in the form of its oxides or phosphates. Eccentric disk mills and annular chamber mills are preferred as mills. Calcination takes place in the temperature range from 200.degree. to 900.degree. C.
In all of the hitherto known catalysts of this kind, it is essential to make sure that the components (A) and (B) (for example, A=Ti; B=W, Mo etc.) are present as an intimate mixture in the form of their oxides. This mixture is then subjected to a shaping process from which one can obtain the catalysts by pressing or extruding in the form of bulk material or monoliths in honeycomb form.
As a result, the steps of catalyst shaping, for example, extrusion shaping, must be preceded by a process aimed at the formation of the intimate mixture as initial material. This currently customary procedure entails the following disadvantages;
technically expensive, multi-stage, energy-intensive procedural steps are necessary;
the production of the intimate mixture according to German Patent Application 24 58 888 is an environmental hazard; air exhaust and waste water problems appear;
the operation of mills for grinding activation is energy-intensive and demands expensive noise protection measures and dust protection installations;
the pre-treatment process steps for the formation of the intimate mixture are a serious, negative cost factor in catalyst production.